The characteristics of ring spun yarn are unmatched by the products of other yarn spinning techniques which may be more productive, so it is a technology that is unlikely to be superseded in the near term. Much research has gone into ring spinning technology and relatively modest improvements to the productivity of a ring spinning frame can be very significant in a spinning mill where many thousands of spindles are employed. Many different raw material and machine-related factors directly influence both productivity and quality, and sometimes improvements in one area have consequences that necessitate a trade-off in another area. The goal of spinning technologists might therefore be considered a quest for an optimum balance between higher productivity and desired quality.
U.S. Pat. No. 3,979,894 describes a five-belt false twisting device for texturing filament yarn which, with its continuous lengths of filament, has quite different structure and properties to short staple ring spun yarn, and of course, it is not a ring spun yarn. Moreover, in this old false twisting device the filament yarn sequentially passes across parallel runs of five travelling endless belts, wrapping about convex surfaces of each run before passing between adjacent runs. Three of the belts turn in one direction, while two turn in an opposite direction, the filament yarn passing through the runs in a zig-zag manner such that friction between each of the five runs and the filament yarn imparts the false twist in a common direction. The belts are matched and all five belts travel at the same speed, to avoid twist variation that would lead to poor quality of the textured filament yarns and instability of the yarn path.
With respect to ring spinning technology, a more relevant apparatus is described in US2010/0024376, which teaches a single-belt method for imparting false twist to a yarn delivered from the nip of a pair of front drafting rollers immediately before ring spinning. The yarn received from the drafting rollers is drawn generally transversely and sequentially across first and second parallel runs of a single belt, passing about the first run, then between the runs, before passing about the second run. Friction between the first and second runs and the yarn imparts the false twist. The two runs of the belt move in opposite directions, but the linear speeds of the two runs are the same. Compared to conventional ring spinning, at a given production rate this single-belt false twist method produces yarns having lower residual torque which endows a resultant fabric with a softer handle, and it also provides satisfactory strength and reduced hairiness. The single belt can extend the length of a machine, making it a more cost-effective investment than alternative technologies involving heat treatment to reduce residual torque. However, it has been found that this single-belt technology results in an increase in yarn defects above the usual level—including the number of thick places, thin places and neps. A sharp increase in the number of neps is of particular concern, since neps can be a cause of ends down in downstream processing and they may not take up dye like the rest of the yarn, detracting from the appearance of the fabric. Achieving satisfactory nep counts therefore necessitates relatively higher maintenance costs to mitigate machine factors, such as wear, that are known to contribute to nep formation. It will therefore be understood, that a need exists for an improved false twist method and apparatus that is able to at least maintain the above-mentioned advantageous properties while reducing nep formation in ring spinning of short staple yarns. It is an object of the invention to address this need or, more generally, to provide an improved method of imparting false twist to yarn between drafting and ring spinning processes.